It is common practice for pilots of gliders, sailplanes, motorgliders, ultralight airplanes and other relatively low power-to-weight aircraft to seek upward flows of air, and conversely, to avoid its downward flows. This reduces or simply eliminates reliance on an engine to stay aloft for long periods of time. In motorless aircraft, it allows climbing to great heights and it is the basis of the art of soaring.
One of the most common sources of upward flows are found in so-called thermals, when a bubble or a plume of air at a lower density than its surroundings creates an updraft by buoyancy. A glider pilot will navigate and execute search patterns to locate such a thermal, then find its fastest rising core and circle within it to climb, generally by using the visual or aural indications of an aircraft instrument called a variometer, which is an advanced form of Vertical Speed Indicator. The most difficult task, however, is not to circle within an identified thermal, but to find the rising air in the first place.
This task can be critical at low altitude, where the diameter of a thermal is often narrow, and where seeking the thermal in a wrong direction can then make an unwelcome off-airport landing imminent. Furthermore, a thermal is practically always surrounded by significant downdrafts of sinking air, which are undesirable areas to linger in while searching for the thermal itself. The problem is even more difficult when the thermal, sometimes called a "blue" or (inappropriately) "dry" thermal, is not capped by a telltale cumulus cloud to hint at its location. This is often the case early morning or late afternoon when thermals do not have enough energy to reach condensation altitude. Consistently finding thermals on a cloudless day is still a challenge for glider pilots, and few instruments can help in this task.
In searching for a thermal, a skillful glider pilot is constantly on the watch for a wingtip suddenly lifting more than the other, possibly indicating the presence of a thermal boundary, so as to immediately turn towards it. Instrumental detection of this increased lift of one wing by the use of strain gauges measuring the bending moment of the wings is the basis for U.S. Pat. No. 4,506,847 (Norman 1985). However, air at the boundary of a thermal is often turbulent, a source of false positives, and the device may not reveal more to the pilot than what can be observed visually or felt through the aircraft attitude and controls.
Another instrumental method for locating thermals is based on the observed fact that they often consist of air that is warmer, i.e. lighter, than the surroundings. The difference of temperature between wingtips can therefore give indication of where a thermal is relative to the trajectory of the glider. This principle is the basis for U.S. Pat. Nos. 3,798,971 (Lowrance, 1974) and 4,591,111 (Laughter, 1986). However, the generally accepted notion of a "thermal" being warmer than its surroundings is misleading. A bubble or a column of air warmer than its surroundings at ground level will cool by expansion while ascending, sometimes down to a temperature actually cooler than its surroundings, and yet pursue its upward motion by momentum. Abovesaid thermal detection devices based on temperature measurements would then steer the pilot away from the rising air, which is exactly the opposite of the objective of such devices.
It is somewhat less generally known that water vapor is about 62% lighter than dry air, and therefore that humid air will tend to rise by buoyancy in drier air just as warm air rises in cooler air around it. Helmut Reichmann (1993) says that "unusually high local humidity can cause such localized phenomena as thermals over swamps or even small lakes". The same author states that temperatures in such "moist thermals" are sometimes lower than those of the surrounding air. In this case, again, thermal detection only based on temperature measurements would not only miss the thermal, but lead the pilot away from it. Moist thermals are also nicknamed "wet thermals" although they do not contain water in liquid form, except within clouds. Moist thermals may be less powerful than "warm thermals" and consist in air rising slowly and relatively smoothly. As a result, they may not be easy to detect with the abovesaid methods based on wing strain.
Conceptually, the sun can first evaporate dew and surface water and initiate a moist thermal, yet maintain the ground relatively cool by evaporation, and second, the sun can heat the ground sufficiently to warm up the air above it and initiate a warm thermal. In some conditions, moist thermals may also be produced when a localized light rain evaporates, sometimes before reaching the ground. In this case the buoyancy provided by the resulting increase of atmospheric water vapor content can exceed the sink caused by evaporation cooling and by the downwards air entrainment caused by the remaining falling droplets. The fact is that there are very few comprehensive studies of the structure of thermals at the altitude, speed, and resolution of interest to glider pilots. Published observations are either too low and too static from ground towers, or too high and too fast from instrumented power planes.
Measuring the humidity of ambient air around an airplane is not new. Atmospheric moisture is one of the key elements in meteorological assessment and weather forecasting. Many air sounding aircraft are equipped with a combination of several types of sensors, such as Lyman-alpha hygrometers for absolute humidity, chilled-mirror thermoelectric hygrometers for dew-point, microwave resonance cavities for measuring microwave refracting index, hot-wire evaporators for cloud liquid-water content, among others. This information is collected for weather forecast purposes, and is not used for the management of the flight itself. Humidity of ambient air is also important in the evaluation of the risk of aircraft icing when flying in clouds, or for some humidity sensitive military payloads (U.S. Pat. No. 5,050,109). In this case an alarming humidity reading may prompt the crew to fly at a different altitude to avoid icing. These methods generally provide an actual humidity reading that is as accurate as possible.
Humidity is a variable which is inherently more difficult to measure than temperature. Not only is there a very wide variety of humidity measuring devices, but there are many ways of expressing humidity itself, such as mixing or mass ratio, specific humidity, absolute humidity, volume ratio, partial pressure, dew point, relative humidity, etc. Humidity air soundings showing evidence of moist thermals are generally expressed in terms of specific humidity (g of vapor per kg of vapor and dry air mix) or in terms of the numerically quite close mixing ratio (g of vapor per kg of dry air). The majority of common humidity sensors provides an output in terms of relative humidity (RH), which is the ratio between actual water vapor partial pressure and saturation water vapor pressure at the same temperature. Relative humidity depends heavily on temperature by definition.
Calculations show that by using plain relative humidity sensors to detect thermals, the pilot of a glider would be unable to ascertain if a change of RH was due to an actual change of humidity (RH increase in moist thermal), a change of temperature (RH decrease in warm thermal), or whatever combination of the two. To subtract the effect of temperature on RH, the common practice is either to artificially heat the sensor, which may also accelerate chemical reactions in some sensors (Figaro 1997), or to collect temperature information alongside RH so that the actual water vapor pressure and derived units can be mathematically computed (Nowak 1996).